Clean up CI, add c8_air_shower to pipeline

Firstly, this includes a clean up of the CI instructions yaml file.

• more uniform formatting
• use of absolute directories instead of relative ones

Secondly, this adds c8_air_shower to the build-test-examples stage and the python outputs are now run on this.

The code approval procedure is described in the wiki: Code approval procedure wiki

• The MR is without WIP/Draft status
• Make sure the most recent CI jobs (config, quality, build_test_example) all run fine with no failures
  • if "check clang-format" failed: the code contributor has to run ./do-clang-format.py --apply eventually with the --all option
  • if "check copyright" failed the code contributor has to run./do-copyright.py --add=20xy
• Make sure also the jobs with MR-label ready for code review succeed. This includes the optional jobs, in particular 'coverage', 'release-clang-8", "release-u-18.04" and make sure no problems occur. You may have to trigger a pipeline manually to check this.
• Check in the "coverage" job output that the coverage did not decrease. It should always stay, or increase. If it decreased --> ask contributor to add further needed unit tests, and check coverage report.
• On the MR page, open the "Open in Web IDE" tool
  • Check if the provided solution solves the Issue, discuss on gitlab
  • Check that all changes are actually related to the issue
  • There are no debug statements left, not even commented out
  • Check all changes for coding rules and guidelines
• When all above is done:
  • Add MR label "Code Review Finished"